(a) Field of the Invention
The present invention relates to an electroluminescent (referred to as “EL” hereinafter) display and driving method thereof.
(b) Description of the Related Art
An organic EL display is a display emitting light by electrically exciting a fluorescent organic material, and it displays images by voltage-driving or current-driving M×N organic luminescent cells. An organic luminous cell includes an anode (ITO), an organic thin film, and a cathode layer Metal. The organic thin film is formed of multiple-layers including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for improving light-emitting efficiency by balancing electrons and holes, and also includes separate an electron injecting layer (EIL) and a hole injecting layer) (HIL).
The organic luminescent cells are driven by a simple matrix (or passive matrix) type and an active matrix type using thin film transistors (TFTs). The simple matrix driving is to select cathode lines and anode lines crossing each other, while the active matrix driving to connect TFTs and capacitors to ITO pixel electrodes and to store voltages into the capacitors.
FIG. 11 is a circuit diagram of a conventional pixel circuit of a representative one of N×M pixels, for driving an organic EL device using TFTs. Referring to FIG. 11, the organic EL device OELD is connected to a driving transistor Mb for supplying light-emitting current. The amount of current driven by the driving transistor Mb is controlled by data voltage supplied through a switching transistor Ma. A capacitor C for keeping the supplied voltage for a predetermined time is connected between a source and a gate of the transistor Mb. A gate of the transistor Ma is connected to the n-th scan line, and the source thereof is connected to a data line.
Seeing an operation of a pixel with the structure, a selection signal applied to the gate of the transistor Ma turns on the transistor Ma, and then the data voltage VDATA is applied to the gate A of the current driving transistor Mb through the data line. Then, the current flows into the organic EL device OELD through the transistor Mb in response to the data voltage VDATA applied to the gate of the transistor Mb, and the organic EL device OELD emits light.
The amount of the current flowing in the organic EL device is given by Equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                      V                    TH                                                  )                            2                                                          (        1        )            where IOLED is a current flowing in the organic EL device, VGS is a gate-source voltage of the transistor Mb, VTH is a threshold voltage of the transistor Mb, VDATA is a data voltage, and β is a constant.
According to Equation 1, the current supplied to the organic EL device depends on the applied data voltage VDATA in the pixel circuit shown in FIG. 11, and the organic EL device turns to be luminescent in response to the supplied current. Here, the applied data voltage VDATA has multiple values in a predetermined range.
However, the conventional pixel circuit has a drawback in that it causes the non-uniform brightness of the panel because of the characteristic deviation of the thin film transistors caused by the unevenness of manufacturing process.
To compensate for this problem, it is proposed to use additional thin film transistors in a pixel circuit. In this pixel circuit, however, a aperture ratio of the panel decreases due to the increase of the number of the thin film transistors and it takes so long time to charge the capacitor for low gray scale.